1. 化学組成で探る天の川銀河の進化 辻本拓司 ( 国立天文台 )  prompt Type Ia supernovae (2006~)  the IMF variation (classical issue)  stellar migration (2008~)  He-enriched stars in globular clusters (2004~) ４つのテーマに焦点 天の川銀河研究会２０１２, ９月６ − ８日 in 鹿児島  Galactic thick/thin disk  Galactic bulge  Galactic halo  the Fornax dSph galaxy

2. Totani et al. 2008 Hachisu et al. 2008 double-degenerate scenario single-degenerate scenario Kirby et al. 2011 theoretical modelsobserved results from SN Ia surveys for extragalaxies (Totani et al. 2008; Maoz et al. 2010) about 70% of SNe Ia explodes with a time delay within 1 Gyr Young progenitors for SNe Ia are dominant (Mannucci et al. 2006; Sullivan et al. 2006) a significant impact on Galactic chemical evolution Delay Time Distribution (DTD) of SNe Ia Ia 型超新星の寿命は短かった t Ia ~t GW a 4 f sep a -1

3. Drastic change in typical timescale of SN Ia progenitors ~1 Gyr ~ 0.1Gyr [Fe/H] Pagel & Tautvaisiene 1995 break in [a/Fe] among solar neighborhood stars Yoshii et al. 1996 Toomre diagram Venn et al. 2004 thin disk no break no high  /Fe stars no low Fe/H stars thick disk the presence of break! Galactic stars are now well kinematically separated. imply apply DTD discussion on  /Fe break should be assessed by comparing the modeled chemical feature of the thick disk with the corresponding observed one. T=(U 2 +V 2 ) 0.5

4. Updates of s-process yield in AGB stars s-process 元素はもう一つの元素 ( 宇宙 ) 時計 Mainly due to large uncertainties in convective mixing and 13 C-pocket efficiencies, the s-process nucleosynthesis allows a wide range for the level of a possible production. Abundances of the surface of AGB stars can be directly compared with the nucleosynthesis results. the best empirical Ba yield as a function of stellar [Fe/H] so as to reproduce the Ba evolution of thin disk stars The renewed picture of a SNIa clocking should be tied up with another nucleosynthesis clock, the s-process operating in an AGB star. Busso et al. 2001 fails to reproduce the chemical evolution of the thin disk TT & Bekki 2011 Theoretically allowable range

5. Chemical Evolution of Disks ✓ Thick and thin disks are separately modeled. ✓ First, thick disk is rapidly formed, and subsequently thin disk is gradually formed. ✓ Formation of two disks are connected in a sense that thin disk stars start forming from a remaining gas of thick disk. ✓ We examine the evolution of [Mg/Fe] and [Ba/Mg]. not [Ba/Fe] so as to prevent the effect of s-processing from being hidden by SN Ia contamination ✓ We focus on the chemical evolution for [Fe/H] ≤ 0. since the origin of metal-rich disk stars should be assessed with an extra evolution factor such as stellar migration see next slide three nucleosynthesis clocks: SNe II, SNe Ia, AGB stars

6. Formation of the thick disk through minor merging between the first generation of the Galactic thin disk and a dwarf galaxy about ~10 Gyr ago (Bekki &TT 2011, ApJ, 738, 4) no metallicity gradient in the thick disk flattening of metallicity gradient resulting from radial mixing induced by minor merging obs. model The thick disk can be regarded as a first disk which is heated up by an ancient minor merger, that is subsequently followed by the gradual formation of a secondary disk, i.e., the thin disk. Kinematic properties can be also reproduced. Allende Prieto et al. 2006 (but not a positive correlation.. : Spagna et al. 2010; Lee et al. 2011 )

7. Chemical evolution of the thick disk A successful reproduction of the [Mg/Fe] feature suggests that a new SN Ia DTD revealed by extragalaxy studies is compatible with the Milky Way case. model parameters = 2 Gyr -1 : SFR coefficient  SF = 1.5 Gyr: SF duration t in = 0.5 Gyr: timescale of an infall an indication of pre-enrichment including the s-processing due to enriched bulge winds ? or by s-process elements from fast-rotating massive stars ? (Pignatari et al. 2008; Chiappini et al. 2011) data from Bensby et al. 2005 Ruchti et al. 2011 Venn et al. 2004 data from Bensby et al. 2005 Venn et al. 2004 TT & Bekki 2012

8. The thin disk stars start forming from the thick disk's remaining gas (corresponding to ~10 % of the original gas) mixed with the infall gas accreted onto the disk. Chemical evolution of the thin disk model parameters = 0.4 Gyr -1 : SFR coefficient  SF = 12 Gyr: SF duration t in = 5 Gyr: timescale of an infall [Fe/H] and [Mg/Fe] decreases and increases, respectively, owing to dilution by metal-poor infalling gas from the halo. This reverse evolution comes to an end when the chemical enrichment by star formation exceeds the effect of gas dilution, and subsequently an usual evolutionary path appears. the absence of metal-poor thin disk stars as observed the low Ba yield case TT & Bekki 2012 data from Bensby et al. 2005 Venn et al. 2004 data from Bensby et al. 2005 Venn et al. 2004

9. The Galactic Bulge the presence of two populations a bar-like kinematics an old spheroid or a thick disc two distinct populations Babusiaux et al. (2010) studied the correlation between kinematics and metallicities in Baade’s Window. vertex deviation two-peaked MDF red clump stars in Baade’s window Hill et al. 2011 microlensed dwarf and subgiant stars Bensby et al. 2011 [Fe/H] 0

10. Formation of the Galactic bulge from a two-component stellar disk (Bekki &TT 2011, MNRAS, 416, L60) cylindrical rotation vertical metallicity gradient The first disk is disturbed by an ancient minor merger, which induces a vertical growth of the disk and transforms it into a thick disk, and subsequently the thin disk starts to form with an accompanying bar formation in the central region. Two-component scenario A vertical mixing induced by a bar buckling functions incompletely in a sense that the high latitude region in the thick disk is not well mixed. simultaneous reproduction Note that in general, it is expected that a disk instability forming the bulge induces a vertical mixing, which leads to erasing a metallicity gradient along a minor axis.

11. 銀河系バルジの起源は？ merger-built? YES bar-induced? YES では、混在？ metallicity gradient cylindrical rotation single population [Fe/H]=-0.03 [Fe/H]=-0.17 [Fe/H]=-0.28 NO Howard et al. 2009 Minitti et al. 2005 Zoccali et al. 2008

12. metal-poor component  = 4 Gyr -1  SF = 1 Gyr t in = 0.3 Gyr IMF: x = -1.35 metal-rich component  = 3 Gyr -1  SF = 4 Gyr t in = 1.5 Gyr IMF: x = -1.05 ✓ A top-heavy IMF is indispensable to make a metal-rich MDF as observed. ✓ A large age span of bulge stars is predicted. two-component bulge model an end result of chemical processing associated with the halo formation or the s-processing in massive stars ?? ✓ the enriched gas Bensby et al. 2011 TT & Bekki 2012 data from Bensby et al. 2011 Gonzalez et al. 2011 data from Hill et al. 2011 Bensby et al. 2011 data from Bensby et al. 2011 but, the color-magnitude diagram is …

13. one-component bulge model = 2 Gyr -1  SF = 2 Gyr t in = 0.3 Gyr IMF: x = -1.05 x=-1.35 The predicted [Ba/Mg] exhibits a sharper rise from a much lower metallicity than is expected from the observation. This incon- sistency is resolved by the model with a flatter IMF. The MDF with a Salptere IMF is entirely skewed to a low metallicity. In addition, the predicted [Mg/Fe] is lower than the observed data in a metal-rich regime. A top-heavy IMF is suggested. red giants in Baade window (Fulbright et al. 2006; Zoccali et al. 2008) TT & Bekki 2012

14. Halo vs. short-delayed SNe Ia no indication of SNe Ia for the elemental abundance of halo stars, exhibiting a plateau of [  /Fe] ratio over a whole metallicity range Halo stars must be rapidly formed in the Galactic building blocks with a short timescale (~10 8 yr), while an assembly of them finally makes the stellar halo which exhibits an age span of a few Gyr. The s-process elements from AGB stars starting to release with a timescale of a few 10 8 yr are imprinted in the abundances of halo stars?? implies Roederer et al. 2010 s-process: no s-process: yes The compatibility of the presence of s-process among halo stars with the short-delayed DTD can be also understood consistently if s-process elements are produced in fast- rotating massive stars (Pignatari et al. 2008; Chiappini et al. 2011). But,….. to check if it is likely or not…. [La/Eu] continues to make a plateau [Pb/Eu] shows an upward trend with an increasing [Fe/H]

15. Unusual elemental feature of dwarf spheroidals low  /Fe ratio high s abundance in the Fornax dSph [Fe/H] previous results from six dSphs Venn et al. 2004 the LMC, too! Pompeia et al. 2008[Fe/H] Letarte et al. 2010 also seen in the Sagitarrius dSph (Sbordone et al. 2007)

16. previous study proposed idea [Fe/H] GC’s data (Letarte et al. 2006) 810502 stellar mass (M  ) s-process r-process Fe &  elements cut-off Letarte et al. 2010 strong galactic wind model Lanfranchi et al. 2008 [Fe/H] occurrence of winds stop the SF no more r-process Eu from SNe II delayed s-process Ba from AGB stars ~1.5-3 M  ~25-30 M  r-process Ba ([Ba/Eu]~-0.7)

17. Meurer et al. 2009 A truncated IMF in dwarf galaxies observationally theoretically Low-surface brightness galaxies have A low ratio massive O-type stars less massive stars a smaller number of very massive stars A high mass end of the IMF depends on the mass of the star clusters. In the low density environment, the formation of massive star clusters is suppressed. Kroupa & Weidner 2003, Pflamm-Altenburg & Kroupa 2008 Their model predictions have been shown to be consistent with the observed trend for the H  -to-FUV flux ratio (Lee et al. 2009).

18. Model result 3 Mo 1.5 Mo Coleman & de Jong 2008 stars with age > 10 Gyr Cescutti et al. 2006 Busso’s results no metallicity dependence for low metallicity Ba s-process yield Fnx dSph case: M u =25 M  with Δ SF =1.5 Gyr A rapid enrichment is implied by the Galaxy Fnx dSph TT 2011

19. 惑星を持つ太陽近傍星は metal-rich Santos et al. 2003 惑星を持つ星の ほとんどが [Fe/H]>0 の metal-rich な星。 これまで化学進化の分野では、ほとんど無視されていたが …….

20. Nordstrom et al. 2004 metal-rich な星は太陽近傍 の化学進化の終着点 およそ２０％の星が太陽より metal- rich 時間 重元素量 基本的に重元素は時間と ともに増えていくもの 時間 ? 化学進化の描像 0 重元素量 (metallicity) 時間 Cepheids OB stars have [Fe/H]~ 0 H II regions ではなさそう。。 ([Fe/H]>0)

21. 重元素量頻度分布 (ADF) からわかること [Fe/H] present =+0.03 [Fe/H] present =+0.4 the key features of the ADF ・ the deficiency of metal-poor stars ー the G-dwarf problem ー ・ [Fe/H] peak = -0.2 ~ -0.1 & ・ [Fe/H] present > +0.2 The predicted ADFs are biased to metal-rich as compared with the observations. Tsujimoto 2007 Metal-rich な星は簡単には作れない Metal-rich な星は進化の終着点 とは考えにくい。 +0.4 まではありそう ([Fe/H]~+0.3-0.4)

22. Stellar migration (Radial mixing)I stellar migration due to resonant scattering with transient spiral arms explains a large scatter in age-metallicity relation Sellwood & Binney 2002 Roskar et al. 2008 Two-dimensional histogram of final particle radii vs. particle formation radii initial final

23. Stellar migration (Radial mixing)II stellar migration due to resonant scattering with transient spiral arms Sellwood & Binney 2002 Roskar et al. 2008 predicts a steepening of abundance gradient reproduce metal-rich stars w/o migration w/migration present past

24. 金属量勾配の時間変化 I 金属量勾配の時間変化 I Daflon & Cunha 2004 Maciel et al. 2006 obs. GCE models abundance gradient from R~4 kpc to R~14 kpc -0.1 dex kpc -1 -0.04 dex kpc -1 in the last several Gyr predict a steepening (Chiappini et al. 2001) or a flattening (Hou et al. 2001) The predicted change in abundance gradient in the last several Gyr is <0.02 dex kpc -1 - too small - flattening 現在

25. Yong et al. 2006 old stars young stars time 金属量勾配の時間変化 II 金属量勾配の時間変化 II TT, Bland-Hawthorn, & Freeman 2010 銀河系を見る限り、 steepening の証拠はなさそう ( 過去ほど、勾配が緩やか ) old open clusters Cepheids

26. Piotto et al. 2005 double main-sequence in  Cen double main-sequence in  Cen Bedin et al. 2004 blue MSred MS the origin of bMS 1. Y~0.4 2. very low metallicity ([Fe/H]<-2) bMS stars are more metal-rich than rMS stars. f bMS ~ 0.2-0.3 photometric result spectroscopic result  [Fe/H]~0.3 HST reveals VLT reveals

27. super helium-rich stars in GCs super helium-rich stars in GCs Y=025/0.32/0.38 NGC 2808 Milone et al. 2012 47 Tucanae ✓ GC is not a single population! (note: a spread in [Fe/H]:  Cen, M22, Terzan 5, NGC 2419 a subgiant-branch split: NGC 1851, M22, 47 Tuc,…..) ✓ the presence of super He-rich stars only in massive GCs?  >17% halo stars from disintegrated GCs (Martell et al. 2011)  seen for metal-rich stars in the bulge (Nataf & Gould 2012) the origin of He-rich stars likely, the formation from AGB ejecta Milone et al. 2012 NGC 6566

28. まとめ  大きく認識を変えたいこと ✓ Ia 型超新星は結構早く爆発する ~1 億年 ✓球状星団（の一部）は単一の星の種族ではない ヘリウム過剰星の存在  今後注視していきたいディスク銀河進化のドライバー ✓ stellar migration ( 星の動径方向の大移動 ) 但し、過大評価の可能性もある  IMF の普遍性問題 ✓バルジ、近傍矮小銀河に IMF variation の証拠 top-heavytop-light